To get the medium achievement: Create 2 blue towers. upgrad only damage. place a rocket tower near them. upgrade damage and range ( if necessary) this will give you a nuke combo. (its my favorite) keep copying the nuke tower. After a couple of them 3/4 you will get a Combo tower. place it near the nuke tower. if it could do a combo with the 3 towers from the nuke. it wwill change colours between blue and red. blue will be a "Big Landmine Combo" red will be "nuke combo" if you do this a couple of times, you will have towers that only shoot nukes. If you want you can do other combos to. Read http://onslaught.playr.co.uk/about.combos.html for more info about towers. always make sure the turret that should start the combo has the biggest range. and when changing them in a combo tower, make sure it has the right color.
PS: press C to see which combos are available.
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